lint: warn about extern declarations outside headers

https://mail-index.netbsd.org/tech-userlevel/2023/03/15/msg013727.html

tests/lint: guard against typos in test environment configuration

Some tests had "lint1-only-if" without a trailing colon, others included
the trailing colon. The tests that included the trailing colon were run
even though they were supposed to be skipped, and they failed, as could
be expected.

To prevent further typos, always require the trailing colon, just as in
"lint1-flags" and fail fast on unknown "lint1" comments.

lint: fix wrong warning about out-of-range value '\xff' for char

This only affects platforms where char has the same representation as
unsigned char.

lint: document where to fix the wrong warning for '\xff'

tests/lint: add test that only runs where char == unsigned char

There a 4 regular NetBSD builds where lint is activated. All these
builds run on platforms where char == signed char.

The official test runs from https://releng.netbsd.org/test-results.html
mostly have char == signed char as well.

However, lint behaves differently on platforms with char == unsigned
char. On these platforms, a simple "char ch = '\xff'" leads to the
bogus warning that "conversion of 'int' to 'char' is out of range".